The present invention relates to a process for the treatment of petroleum charges and in particular to the dewaxing of hydrocarbons with an improvement to the cold behaviour of petroleum fractions or cuts, e.g. of a gas oil. The invention also relates to the novel catalyst used.
This novel catalyst contains a zeolite from the MFI group or a silica with a structure close to that of MFI, particularly suitable for the selective elimination of normal or slightly isomerized waxes from a petroleum charge.
In efforts to increase the production of light products at the expense of the residual fuel, the refiner seeks to increase the final distillation point of the distillate and the gas oil. However, besides the need not to exceed a certain sulphur content, the major difficulty is the cold behaviour of the gas oil.
These are in fact the characteristics which, most of the time, limit the production of gas oil or domestic fuel oil. They are defined by two standards:
AFNOR no. T 60 105 for the flow point (F.P.) and the turbidity point (T.P.); PA1 AFNOR no. M 07 042 for the filtrability limit temperature (F.L.T.).
The turbidity point in .degree.C. is the temperature at which the first wax crystals appear, whilst the flow point corresponds to the complete setting or solidification of the product. The limit filtrability temperature, intermediate between the two preceding characteristics, is linked with the "pumpability" of the gas oil through a filter under standard conditions.
It is universally accepted that the problems caused by the cold behaviour of gas oils are linked with their content of longchain, linear waxes, whose content generally varies between 10 and 40% by weight.
One of the ways of making it possible to improve the flow properties of gas oils consists of incorporating additives into them, but this method has limited effects. Although it makes it possible to modify the filtrability point and the flow point, it is known to have little effect on the turbidity point. In addition, it should be noted that the action of additives is sometimes very small on certain crude oils.